Measured jet plume static pressure distributions and far-field acoustic spectra are presented for underexpanded jets in simulated flight up to a Mach number of 0.4. A gradual stretching of the downstream shock cells is found as the Mach number increases, with no perceptible change in the shock strength. There appears to be little effect of flight on the broadband shock noise amplitudes, and the small changes in its peak frequency for the same emission angle are correlated with the slightly longer shock cells in flight. The larger changes in the broadband peak frequency found at the same angle in wind tunnel coordinates are attributable to convection. Jet mixing noise production decreases significantly with increasing flight speed.